In recent years, quantum key distribution (QKD) as a method for enabling cryptographic communications that are information-theoretically secure against wiretapping is actively researched (NPL 1), and development aiming at practical use of QKD is advancing.
In QKD, a “single photon”, which only includes a single photon per pulse, (or a “pseudo single photon”, which is obtained by attenuating a general laser beam to the utmost extent) is used as a communication medium, and therefore, a photon detector capable of detecting single photons is used instead of a photodetector as one employed in general optical communications. As such a photon detector, an avalanche photodiode (APD), to which a bias exceeding a breakdown voltage is applied, or a superconducting device cooled to several K is generally used.
Although various QKD systems have been suggested, a general system is the one in which two or four photon detectors are used (refer to NPL 2, for example). In the case of employing multiple photon detectors, it is desired that the characteristics of all the photon detectors are as uniform as possible in order to guarantee the security of a cipher key generated by QKD, however, in general, the characteristics of APD devices and superconducting devices vary greatly and also change due to, for example, environmental temperature fluctuation and deterioration of the devices. Accordingly, in the case where QKD operates over a long time period, it is necessary to equalize the characteristics by regularly checking the characteristics and individually adjusting external parameters such as bias voltages.
Examples of major parameters representing the characteristics of a photon detector are quantum efficiency and dark count probability. Quantum efficiency is a probability that a photon detector correctly outputs a detection signal upon receipt of a pulse including a single photon (photon detection probability). Dark count probability is a probability of erroneously outputting a detection signal although no photon is included, and represents the magnitude of noise. Typical values of quantum efficiency and dark count probability are approximately 10% and 10−5, respectively.
As a method of equalizing the characteristics of multiple photon detectors, there is a method of estimating the difference in quantum efficiency on the basis of the variance in measured detection data and adjusting bias voltages or the like of the detectors so as to eliminate the variance (PTL 1). FIG. 10 is a block diagram illustrating this technique. This optical receiver includes: multiple photon detectors 201 (two, i.e., 201a and 201b, in this example); a bias voltage control means 202, which controls bias voltages of the photon detectors 201; and a detection number determining means 203, which counts the number of photons in an output from each of the photon detectors 201. In this device, when signal beams 204a and 204b are input and a variance in the number of detections between the two photon detectors 201a and 202b is found, the bias voltage control means 202 adjusts the bias voltages of the two photon detectors 201, consequently eliminating the variance. In other words, the quantum efficiencies of the multiple photon detectors 201 may be equalized.